Production of transgenic rabbits using centrifuged pronuclear zygotes

Superovulation with a single administration of FSH in aluminum hydroxide gel: a novel superovulation method for cattle

Superovulation (SOV) is a necessary technique to produce large numbers of embryos for embryo transfer. In the conventional methods, follicular stimulating hormone (FSH) is administered to donor cattle twice daily for 3 to 4 days. As this method is labor intensive and stresses cattle, improving this method has been desired. We previously developed a novel and simple SOV method, in which the intramuscular injection of a single dose of FSH in aluminum hydroxide gel (AH-gel) induced the growth of multiple follicles, ovulation and the production of multiple embryos. Here we show that AH-gel can efficiently adsorb FSH and release it effectively in the presence of BSA, a major interstitial protein. When a single intramuscular administration of the FSH and AH-gel mixture was performed to cattle, multiple follicular growth, ovulation and embryo production were induced. However, the treatments caused indurations at the administration sites in the muscle. To reduce the muscle damage, we investigated alternative administration routes and different amounts of aluminum in the gel. By administering the FSH in AH-gel subcutaneously rather than intramuscularly, the amount of aluminum in the gel could be reduced, thus reducing the size of the induration. Moreover, repeated administrations of FSH with AH-gel did not affect the superovulatory response. These results indicate that a single administration of FSH with AH-gel is an effective, novel and practical method for SOV treatment.

State of actin cytoskeleton and development of slow-frozen and vitrified rabbit pronuclear zygotes

This study was focused on the effect of cryopreservation on the state of actin cytoskeleton and development of rabbit pronuclear zygotes. Zygotes were collected from superovulated females and immediately used for 1) slow-freezing in a solution containing 1.5 M 1,2-propanediol and 0.2 M sucrose, or 2) vitrification in a solution containing 42.0% (v/v) of ethylene glycol, 18.0% (w/v) of dextran and 0.3 M sucrose as cryoprotectants. After thawing or warming, respectively, zygotes were evaluated for 1) actin distribution, 2) in vitro or 3) in vivo development to blastocyst. Comparing actin filaments distribution, a significantly higher number of vitrified zygotes with actin distributed in cell border was observed (55 ± 7.7 vs. 74 ± 6.1% for slow-frozen vs. vitrified, respectively). After 24 and 72 h of in vitro development, significant differences in the cleavage and morula rate among the groups were observed (9 ± 2.4 and 3 ± 1.3 vs. 44 ± 3.0 and 28 ± 2.7% for slow-frozen vs. vitrified, respectively). None of the slow-frozen zygotes reached the blastocyst stage, in contrast to the vitrified counterparts (11 ± 1.9%). Under in vivo culture conditions, a significant difference in blastocyst rate was observed between vitrified and fresh embryos (6 ± 1.5 vs. 35 ± 4.4% respectively). Our results showed that alterations in actin cytoskeleton and deteriorated development are more evident in slow-frozen than vitrified pronuclear zygotes. Vitrification method seems to be a more effective option for rabbit zygotes cryopreservation, although pronuclear zygotes manipulation per se resulted in a notable decrease in embryo development.

Increased transgene integration efficiency upon microinjection of DNA into both pronuclei of rabbit embryos

Transgenic rabbits provide a useful biological model for the study of the regulation of mammalian genes. However, transgene integration efficiency has generally been low. Here we present a first attempt to increase the integration rate of exogenous DNA into the rabbit genome, using a double pronuclei microinjection method. Pronuclear stage rabbit embryos were recovered from superovulated NZW females, 19-20 h after hCG injection. About 5 microg/mL of exogenous DNA solution was microinjected either into one pronucleus (single microinjection, SM) or into both pronuclei (double microinjected, DM). The transgene consisted of a 2.5 kb murine whey acidic protein promoter (mWAP), 7.2 kb cDNA of the human clotting factor VIII (hFVIII), and 4.6 kb that of 3' flanking sequences of the mWAP gene. The in vitro survival of DM embryos to the blastocyst stage was lower than that of SM embryos (68 vs. 89%). Similar results were obtained using EGFP as a control gene construct. However, there was no difference in the percentage of embryos that developed into live offspring using DM (25%) vs. SM (26%). The integration frequency of mWAP-hFVIII into the genome of transgenic rabbits was 3.3% (1/30) upon SM and 8.1% (4/49) at DM (p < 0.05). All founders transmitted the transgene to their offspring in a Mendelian fashion. The SM founder female secreted 87.4 microg/mL rhFVIII in milk, with an activity of 0.594 IU/mL. The DM founder female produced 118 microg/mL rhFVIII, with activity values of 18 IU/ mL. This is the first report of transgenic rabbit production using a double microinjection technique. Our preliminary results suggest that this method can increase the efficiency of production of transgenic rabbit founders, giving a higher integration rate than single microinjection.

Successful vitrification of pronuclear-stage rabbit zygotes by minimum volume cooling procedure

Rabbit zygotes at the pronuclear-stage were cryopreserved by vitrification using a gel-loading tip (GL-tip), Cryoloop or Cryotop. In GL-tip and Cryoloop methods, zygotes were first exposed to 10% ethylene glycol (EG)+10% DMSO in TCM199+20% fetal bovine serum (FBS) for 2 min, and then equilibrated for 30 s in a vitrification solution composed of 20% EG+20% DMSO+0.6 M sucrose in TCM199+20% FBS. In Cryotop method, zygotes were first exposed to 7.5% EG+7.5% DMSO+20% FBS in TCM199 for 3 min, and then equilibrated for 1 min in a vitrification solution composed of 15% EG+15% DMSO+0.5 M sucrose+20% FBS in TCM199. In vitro culture of vitrified-warmed zygotes using GL-tip and Cryoloop resulted in low cleavage rates (2 and 5%, respectively) and no development into blastocysts. In contrast, zygotes vitrified-warmed using Cryotop exhibited higher proportions of cleavage (58%) and development into blastocysts (24%). When compacted morulae or early blastocysts were vitrified by these three procedures, 80-93% of them exhibited blastocoele expansion or zona hatching during the subsequent 48 h of culture. Use of Cryotop instead of GL-tip or Cryoloop for zygote vitrification, without changing conditions of solutions and periods for exposure, equilibration and post-warm dilution, resulted in cleavage and blastocyst development rates of 88 and 45%, respectively. A longer exposure time (10 min) of zygotes to 7.5% EG+7.5% DMSO+20% FBS in TCM199 resulted in higher proportions of zygotes cleaving (94%) and developing into blastocysts (51%) after Cryotop vitrification. Proportions of post-warm zygotes (10-min exposure group) and fresh control zygotes developing into newborn offspring were 36 and 53%, respectively. Pronuclear-stage rabbit zygotes were successfully cryopreserved by vitrification using the Cryotop method.

Effects of cryopreservation of pronuclear-stage rabbit zygotes on the morphological survival, blastocyst formation, and full-term development after DNA microinjection

The objectives of this study were to examine the freezing sensitivity of pronuclear-stage rabbit zygotes and to produce transgenic rabbits using the cryopreserved zygotes. Zygotes were cryopreserved either by one of two vitrification protocols or by one of the two conventional freezing protocols. The morphological survival rates of zygotes subjected to two-step freezing in 1.5 M ethylene glycol and 0.1 M sucrose (74%) or to vitrification in 7.2 M ethylene glycol and 1.0 M sucrose (81%) were higher than those subjected to freezing in 1.5 M DMSO (46%) or to vitrification in a mixture of 2.0 M DMSO, 1.0 M acetamide, and 3.0 M propylene glycol (41%). But the in vitro development into blastocysts of zygotes cryopreserved by vitrification (17%) or to a lesser extent by freezing (52%) was impaired, when compared to that of fresh control zygotes (89%). Next, a fusion gene composed from bovine aS1-casein promoter and a human GH structural gene (2.8 kb) was microinjected into the pronucleus of rabbit zygotes frozen-thawed in ethylene glycol and sucrose. Then, the presence of exogenous DNA in the genome of newborn offspring was determined by PCR. The post-injection survival of frozen zygotes (97%) was the same as that of fresh control zygotes (96%). However, of 18 offspring derived from 414 frozen-thawed and DNA-injected zygotes, no transgenic rabbits were produced. Of 52 offspring derived from 403 DNA-injected fresh zygotes, 3 transgenic rabbits were found. Here we report the first rabbit offspring resulting from zygotes cryopreserved at the pronuclear-stage, although the cryopreservation procedure employed must be improved if zygotes are to be used for systematic production of transgenic rabbits.

A comparative study on the integration of exogenous DNA into mouse, rat, rabbit, and pig genomes

Transgenic mammals, from small laboratory rodents to domestic animals, have been successfully produced to date, but their production efficiency within or across species has been variable. This is probably due to the differences in the type of injected DNA and/or technical procedures employed in each laboratory, as well as the reproductive characteristics of the species. Here we report the direct comparison of the efficiencies of producing transgenic mice, rats, rabbits and pigs by one technician using a fusion gene composed of the bovine alpha S1-casein promoter and human growth hormone (hGH) gene. Before the fusion gene was injected into the zygotes, high magnitude centrifugation to visualize the pronuclei was necessary for all of the pig zygotes and one-third of the rabbit zygotes, but not for mouse and rat zygotes. Post-injection survival of the mouse zygotes (67.1%) was lower than those of the rat, rabbit and pig zygotes (89.6 to 100%). The volume change of the pronucleus following DNA injection was the lowest in mice (50% increase), moderate in rabbits (148% increase), and the most prominent in rats (238% increase). The data from only 1 pig zygote indicated a 22% increase in the pronucleus volume by DNA injection. The PCR analyses of the tail DNA of new born offspring indicated that 0.8% (4/493), 4.8% (22/463), 0.8% (3/367) and 0.9% (2/221) of the injected eggs in mice, rats, rabbits and pigs, respectively, developed into transgenic offspring. Some of the founder animals in all four species expressed the transgene in the mammary gland which was confirmed in hGH mRNA by RT-PCR and/or hGH peptide in Witch's milk with ELISA. These results suggest that the maximum volume of DNA solution injectable into the pronucleus is a possible factor explaining the species differences in the production of transgenic animals.